A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to determine features of the substrate, such as its alignment, a beam is reflected off the surface of the substrate, for example at an alignment target, and an image is created on a camera of the reflected beam. By comparing the properties of the beam before and after it has been reflected off the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties.
To inspect different points on the substrate the substrate must be moved relative to the detector. To inspect every point on the substrate the detector and substrate must be moveable with respect to each other in two directions. Furthermore, due to possible rotational asymmetry in the apparatus it must be possible to detect every point on the substrate from every rotational angle to the substrate, i.e. the detector and substrate must be configured to detect any point on the substrate at any given rotation with respect to each other.
Conventionally the substrate table has been moveable in a first linear direction and rotationally and the detector has been moveable in a second linear direction (perpendicularly to the first linear direction). However, such linear movement requires a lot of space (the substrate table and detector must both be moved by at least 300 mm) and furthermore generates large acceleration forces resulting in significant vibrations.
It is desirable to provide scatterometry apparatus near and amongst the lithographic apparatus in a clean environment. Space in such environments is at a premium and therefore it is desirable to minimize the space occupied by the apparatus.
As is well known accuracy of exposure in the lithographic apparatus is very important and because the scatterometry apparatus are often arranged near the lithographic apparatus it is desirable to minimize the vibrations resulting from the use of the scatterometry apparatus.